It is well known in the art that semiconductor material such as doped silicon possess piezoresistive characteristics. This simply means that the electrical resistance of the semiconductor material changes when the material is subjected to strains such as bending. By attaching a resistive measuring device to the semiconductor material, the change in resistance, and hence the strain applied strain to the semiconductor material, can be measured. There exist a variety of methods known in the art for fabricating piezoresistive sensors from semiconductor materials.
For example, a single piece of doped silicon may be bonded by means of an adhesive gluing process to one side of a strain receiving member. The strain receiving member is typically a flexible metal sheet, bellows or diaphragm. The opposed side of the strain receiving member is exposed to the media that is being measured causing the member to bend, with the strain measured by measuring the change in resistance of the silicon. The major drawback of this "glued gauge" sensor technology is its susceptibility to output drift. As the sensor ages, the bond between the semiconductor material and the strain receiving member also changes thereby requiring the attached resistive measuring electronics for the gauge to be periodically recalibrated.
To obtain linear response from a piezoresistive sensor, it is well known that the formed pressure responsive resistor should be electrically insulated or isolated from the strain receiving member and its support structure. To fabricate such a device as taught by the prior art, monocrystalline silicon is diffused with impurities of one conductivity type and attached to the strain receiving member. The piezoresistive sensor is then formed by subsequently doping monocrystalline silicon with an opposite conductivity type to form an insulating PN junction. Doped monocrystalline silicon exhibits an acceptable gauge factor and has been found well suited for measuring piezoresistive characteristics.
In U.S. Pat. No. 4,003,127 issued to Jaffe, et al., there is disclosed a piezoresistive semiconductor device wherein the strain receiving member is formed from polycrystalline silicon semiconductor material. The same semiconductor material used for the strain receiving member is then also used to form the piezoresistive sensor itself. As opposed to monocrystalline silicon which exhibits a high gauge factor, polycrystalline silicon has a lower, albeit acceptable, gauge factor. The polycrystalline silicon layer, as deposited, has poor conductivity and therefore functions poorly as a piezoresistive sensor. To form the sensor, a masking/etching (photolithography) step is used to define a pressure responsive resistor area in the semiconductor diaphragm material. Through insitu doping, diffusion or ion implantation, followed by activation, the resistor area becomes a piezoresistive sensor. While the fabrication process of Jaffe solves the problems experienced with glued sensors, the strain receiving member formed from the semiconductor material is either exposed directly to the media to be measured, or isolated by means of a secondary diaphragm as will be discussed below.
While the Jaffe, et al. patent discloses and discusses polycrystalline silicon piezoresistive sensors having semiconductor diaphragms, the polycrystalline silicon piezoresistive process disclosed is adaptable for use with strain receiving members of other material types, such as metal. In U.S. Pat. No. 4,657,775 issued to Shioiri, et al., a method for depositing piezoresistive doped polycrystalline films on metal diaphragms utilizing a plasma enhanced chemical vapor deposition (PECVD) process is disclosed. The doped polycrystalline silicon, as deposited, is highly resistive and therefore possesses poor piezoresistive qualities.
To produce piezoresistive measuring devices from the deposited doped polycrystalline silicon on metal layer, photolithography steps are used. For piezoresistive sensors formed by the Shioiri, et al. process, only the area of polycrystalline silicon film on the diaphragm forming the actual gauge pattern is required. Through masking and etching, the gauge pattern is formed and unnecessary semiconductor material removed. After activation of the piezoresistive sensor, a set of metal contacts are applied to the Jaffe, et al. and Shioiri, et al. sensors to enable connection of the electronic resistive measuring circuitry.
The piezoresistive sensor fabrication techniques disclosed by the Jaffe, et al. and Shioiri, et al. patents leave room for improvement. For example, the use of a semiconductor diaphragm, as taught by Jaffe, et al., is undesirable as the sensor cannot be used to measure certain media, for example, corrosives such as acid, that are incompatible with semiconductor material. In addition, the photolithography process utilized by both Jaffe and Shioiri to form the gauge pattern in the deposited polycrystalline silicon material adds to the fabrication cost of individual piezoresistive sensors. Furthermore, the addition of metal sensor contacts after sensor fabrication tends to adversely affect manufacturing costs and increases the number of defective sensors. Accordingly, there is a need for an improved and less expensive method for high volume fabrication of thin film piezoresistive semiconductor sensors exhibiting close design tolerances.